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another ; still less am I forgetful of the fact that the fatality of
an epidemic is largely influenced by the means taken, or the
neglect to take means, to limit the spread of infection. Again,
the rate of mortality in the Metropolis from whooping cough in
1881, was the lowest on record; it is not surprising to find
therefore that in 1882 the disease was excessively fatal.
Diarrhœa may be cited as an illustration of quite another kind.
The mortality from this disease amongst infants was excessive in
1878; the mortality in 1879 was far below the average: but
the diminished mortality in the second year had on relation to
the excessive mortality in the first; the conditions were altogether
different. The summer season in 1879 was cold and wet,
and in a cold and wet summer the mortality from infantile
diarrhoea is always low, just as it is always high when the
summer is hot and dry, as it was in 1878. Per contra, it may
be observed that the conditions that were so favourable to infantile
life in respect of diarrhoea, in 1879, were very unfavourable
to life, both in the young and in the aged, in respect of
another class of diseases—viz., the pulmonary, the mortality
from which throughout the year was excessive, as it always is in
cold and wet seasons.
Again, the significance of a high rate of prevalence of enteric
fever may vary widely in different circumstances: the disease
may be constantly present in one district owing to drainage
defects, or a bad water supply; in another its introduction may
be due to an accidental pollution of water, or to a contaminated
milk supply—and the same observation in respect of milk,
applies equally in the case of scarlet fever also.
These and like circumstances must be kept steadily in view if
we would draw sound conclusions from a high or a low rate of
prevalence of this or that disease, or class of diseases, especially
in relation to the sanitary condition of a district.
Subject to corrections for climatic conditions, and for high
rates in previous years, the concurrence of a low zymotic rate
with a low general death-rate, furnishes just ground for